1. Field of the Invention
This invention relates to a method for producing images, and especially positive-working, contour images from photopolymerizable layers containing nitroso dimers.
2. Description of the Prior Art
In the U.S. application of Pazos, Ser. No. 324,880 and Nacci et al., Ser. No. 324,877, both filed Jan. 18, 1973, and now abandoned, photopolymerizable compositions are described which contain (1) nongaseous, ethylenically unsaturated compound capable of polymerization by free-radical initiated chain propagation, (2) an organic, radiation-sensitive, free-radical generating system, and (3) a thermally dissociable nitroso dimer. These inventions are based on the fact that nitroso dimers are not free-radical polymerization inhibitors, but dissociate to active inhibiting nitroso monomers. Such compounds added to conventional photopolymer compositions thus act as a source of an effective inhibitor for polymerization which extends shelf life and effectively prevents polymerization by thermal initiation. These compositions are used in a single exposure system to give photopolymerized films with greatly improved resolution.
In U.S. application Ser. No. 542,566, filed Jan. 20, 1975, W. J. Nebe describes a two-exposure method of making a positive-working image on a substrate by
A. APPLYING TO THE SUBSTRATE A LAYER OF PHOTOPOLYMERIZABLE COMPOSITION CONTAINING (1) NONGASEOUS ETHYLENICALLY UNSATURATED COMPOUND CAPABLE OF ADDITION POLYMERIZATION BY FREE-RADICAL INITIATED CHAIN PROPAGATION, (2) 0.1-10% BY WEIGHT, BASED ON THE PHOTOPOLYMERIZABLE COMPOSITION, OF A NITROSO DIMER WHICH IS A NONINHIBITOR OF FREE-RADICAL POLYMERIZATION AND IS DISSOCIATED BY SHORT WAVELENGTH UNLTRAVIOLET RADIATION TO NITROSO MONOMER WHICH IS AN INHIBITOR OF FREE-RADICAL POLYMERIZATION, AND (3) 0.001-1.0 PART BY WEIGHT, PER PART OF UNSATURATED COMPOUND, OF AN ORGANIC FREE-RADICAL GENERATING SYSTEM ACTIVATABLE BY ACTINIC RADIATION THAT DOES NOT DISSOCIATE THE NITROSO DIMER TO NITROSO MONOMER,
B. IMAGEWISE EXPOSING A PORTION OF THE PHOTOPOLYMERIZABLE LAYER THROUGH AN IMAGE-BEARING TRANSPARENCY TO ULTRAVIOLET RADIATION THAT DISSOCIATE THE NITROSO DIMER TO NITROSO MONOMER, THEREBY INHIBITING PHOTOPOLYMERIZATION IN THE EXPOSED AREAS,
C. EXPOSING A GREATER PORTION OF THE PHOTOPOLYMERIZABLE LAYER, INCLUDING THE AREAS EXPOSED TO THE IMAGEWISE EXPOSURE RADIATION, TO ACTINIC RADIATION THAT ACTIVATES THE FREE-RADICAL GENERATING SYSTEM BUT DOES NOT DISSOCIATE THE NITROSO DIMER TO NITROSO MONOMER, WHEREBY A POSITIVE POLYMERIC IMAGE IS FORMED IN THE AREAS NOT EXPOSED TO THE IMAGEWISE EXPOSURE RADIATION.
That invention, is based on the fact that nitroso dimers are not free-radical polymerization inhibitors, but are photochemically dissociated to nitroso monomers which are inhibitors of free-radical polymerization by exposure to short wavelength ultraviolet radiation during the imagewise exposure step. The actinic radiation exposure is conducted using radiation that includes wavelengths which activate the free-radical generating system but does not include wavelengths which dissociate the nitroso dimer to nitroso monomer.
During the actinic radiation exposure free-radicals are generated in the area struck by the imagewise exposure radiation just as they are in the other areas. In the area struck by the imagewise exposure, however, the nitroso monomer inhibitor formed by dissociation of nitroso dimer during the imagewise exposure interferes with the normal free-radical induced polymerization process. Accordingly, polymerization does not take place in the area struck by the imagewise exposure radiation.
It is believed that the nitroso monomer inhibits polymerization by reaction with free-radicals or with photoactivated nitroso monomer to form stable nitroxide radicals which do not propagate the free-radical chain process and hence serve as efficient chain terminators. The reactions believed to be operating are outlined in equation 1-3, wherein ##STR1## represents a typical nitroso dimer and RNO* represents a photoexcited nitroso monomer species, ##STR2##
In the context of photohardenable or photopolymerizable systems, a "positive image" in the photosensitive layer is one in which the areas of the layer corresponding to the dark or opaque areas of the original (e.g., a process transparency) are photohardened while the areas corresponding to the light or transparent areas of the original are not substantially photohardened. A "positive image" of this kind provides a "positive-working system". The nonphotohardened material can be removed by means such as solvent wash-out, thereby leaving the photohardened material as a relief on the substrate. The resulting substrate with the so-developed layer can be used as either a relief or planographic printing plate to print true copies of the original.